A sulfonated polyUrylene ether) copolymer, a method of preparing the same, and a polymer electrolyte membrane using the sulfonated poly(arylene ether) copolymer are provided. A sulfonated poly(arylene ether) copolymer containing a sulfonic acid is synthesized by poly condensing a dihydroxy monomer having a sulfonate group with a dihalide monomer, or by polycondensing a dihalide monomer having a sulfonate group with a dihydroxy monomer. Moreover, a crosslinkable dihydroxy monomer or a crosslinkable dihalide monomer is polycondensed with the obtained poly(arylene ether) copolymer, thus enable crosslinking between polymers. A polymer electrolyte membrane for a fuel cell formed using a poly(arylene ether) copolymer containing a crosslinkable moiety maintains the equivalent or superior levels to existing sulfonated poly(arylene ether) polymer or the Nafion membrane commercially available at present in terms of thermal stability, mechanical stability, chemical properties, film formability, and the like, and shows considerably improved proton conductivity and cell performances.